This invention relates to a hydraulic control circuit suitable for use for a vehicle such as a forklift truck or the like and a hydraulic control apparatus for such a hydraulic control circuit, and more particularly to a hydraulic control circuit for positively providing a stable pilot pressure and unloading fluid discharged from a pump to ensure safety of the hydraulic control circuit when any trouble such as non-operation of the circuit due to sticking of a spool of a control valve or the like occurs and a hydraulic control apparatus therefor.
Now, a prior art will be described with reference to FIG. 7 which exemplifies a hydraulic control circuit which has been conventionally used for a forklift truck. The hydraulic control circuit shown in FIG. 7 includes a pump P and a preference valve PV connected to the pump P. The preference valve PV is provided with a control flow port 101, which is connected to a first circuit system S.sub.1. The first circuit system S.sub.1 functions to control a cylinder for power steering. The preference valve PV is also provided with an excess flow port 103, which is connected to a second circuit system S.sub.2. The second circuit system S.sub.2 acts to control an actuator of an implement system. The preference valve PV is adapted to preferentially divert pressure oil in a predetermined or controlled amount or below to &he side of the control flow port 101 and feed pressure oil in an excessive amount exceeding the controlled amount to the second circuit system S.sub.2.
The second circuit system S.sub.2 includes a first control valve V.sub.1 for controlling a lift cylinder, a second control valve V.sub.2 for controlling a tilt cylinder and a third control valve V.sub.3 for controlling an attachment cylinder which are arranged in order from an upstream side. The first, second and third control valves V.sub.1, V.sub.2 and V.sub.3 include spool sections 105, 107 and 109, respectively. The spool sections 105, 107 and 109 are associated at both ends thereof with pilot chambers 111, 113 and 115, respectively, which are arranged so as to communicate through a pressure reducing valve 117 with the pump P. The pressure reducing valve 117 is adapted to determine a maximum value of a pilot pressure acting on each of the pilot chambers 111, 113 and 115.
The first control valve V.sub.1, second control valve V.sub.2 and third control valve V.sub.3 also include proportional solenoids 119, 121 and 123, which function to control pilot pressures acting on the pilot chambers 111, 113 and 115 within a range up to the maximum pressure determined by the pressure reducing valve 117, respectively. Further, the first, second and third control valves V.sub.1, V.sub.2 and V.sub.3 are provided with neutral ports 125, 127 and 129, respectively. When the first to third control valves V.sub.1 to V.sub.3 each are kept at a neutral position shown in FIG. 7, the neutral ports 125, 127 and 129 are open to cause operating or hydraulic oil fed through the excess flow port 103 to the second circuit system S.sub.2 to be returned through a neutral flow passage 131 to a tank T.
The third control valve V.sub.3 is provided on a downstream side thereof with a back pressure valve 133. The back pressure valve 133 is adapted to cause a pressure of a predetermined level to be produced in the neutral flow passage 131 when hydraulic oil flows through the neutral flow passage 131.
For example, when the first control valve V.sub.1 is changed over to a change-over position indicated on a left side of FIG. 7 which is a lowered position of the first control valve V.sub.1, a port on the side communicating with the actuator is caused to communicate with the tank T and the neutral port 125 is caused to be kept open, resulting in the lift cylinder lowering by gravity.
This results in the excess flow port 103 communicating with the tank T; therefore, if the back pressure valve 133 is not arranged, the second circuit system S.sub.2 fails to produce a sufficient pressure on the side of the excess flow port 103. In addition, if a load pressure is not generated on the side of the first circuit system S.sub.1 under such conditions, the hydraulic control circuit fails to generate any circuit pressure. This fails to permit a pilot pressure for changing over the first control valve V.sub.1 to be generated, resulting in a failure of changing-over of the first control valve V.sub.1 or insufficient changing-over of the valve. The above-described arrangement of the back pressure valve 133 at the neutral flow passage 131 is to eliminate the disadvantage.
The first to third control valves V.sub.1 to V.sub.3 include inflow ports, respectively, which are arranged so as to communicate with each other through a parallel feeder 135, so that the control valves V.sub.1 to V.sub.3 are fed with hydraulic oil through the parallel feeder 135. To the parallel feeder 135 is connected an unload valve 137. The unload valve 137 includes a pressure chamber 139, which is arranged so as to communicate through a solenoid valve 141 with the neutral flow passage 131. The solenoid valve 141 is normally kept open to permit the pressure chamber 139 to communicate with the tank T. Thus, when a pressure of the pump acts on the parallel feeder 135, the unload valve 137 is caused to be open, so that pressure oil in the parallel feeder 135 may be returned to the tank T.
The solenoid valve 141 constructed as described above is closed when the proportional solenoids 119, 121 and 123 of the first, second and third control valves V.sub.1, V.sub.2 and V.sub.3 are energized. More particularly, when the first to third control valves V.sub.1 to V.sub.3 each are to be changed over to a position other than the neutral position to actuate the actuator connected thereto, the solenoid valve 141 is changed over to a closed position to interrupt the communication between the pressure chamber 139 of the unload valve 137 and the tank T.
When the communication between the pressure chamber 139 of the unload valve 137 and the tank T is thus interrupted, the unload valve 137 is kept closed even if a pressure is generated in the parallel feeder 135. Therefore, pressure oil which has been fed to the parallel feeder 135 is caused to be fed to the first to third control valves V.sub.1 to V.sub.3.
Now, the manner of operation of the conventional hydraulic control circuit constructed as described above will be described hereinafter.
When the amount of fluid or hydraulic oil discharged from the pump P exceeds a predetermined level, hydraulic in an excessive amount exceeding the predetermined amount is fed to the second circuit system S.sub.2. At this time, when the first to third control valves V.sub.1 to V.sub.3 each are at the neutral position shown in FIG. 1, hydraulic oil is returned through the neutral flow passage 131 and back pressure valve 133 to the tank T. Such flowing of hydraulic oil through the back pressure valve 133 causes a back pressure to be generated. Therefore, when any one of the proportional solenoids 119 to 123 of the first to third control valves V.sub.1 to V.sub.3 is operated under the conditions that the back pressure has been thus generated, a pilot pressure corresponding to the amount of operation of the proportional solenoid acts on each of the pilot chambers 111 to 115 to change over each of the first to third control valves V.sub.1 to V.sub.3. When the proportional solenoids 119 to 123 of the first to third control solenoids V.sub.1 to V.sub.3 are operated, the solenoid valve 141 is concurrently energized, resulting in being changed over from the open position shown in FIG. 1 to the closed position. This causes the unload valve 141 to be closed.
When any emergency occurs while each of the actuators is being actuated, the proportional solenoid of the control valve which controls the actuator is de-energized, resulting in the solenoid valve 141 being also de-energized. This causes the solenoid valve 141 to be changed over to the open position to permit the pressure chamber 139 of the unload valve 137 to communicate with the tank T. When a pressure in the pressure chamber 139 reaches a tank pressure, the unload valve 137 is rendered open, so that hydraulic oil from the excess flow port 103 is returned through the parallel feeder 135 and unload valve 137 to the tank T. This prevents hydraulic oil from being fed to the actuator, to thereby stop actuation of the actuator.
The unload valve 137, when the first to third control valves V.sub.1 to V.sub.3 each are kept at the neutral position, exhibits an additional function of returning hydraulic oil which has flowed into the second circuit system S.sub.2 to the tank T, to thereby prevent a temperature of hydraulic oil in the second circuit system S.sub.2 from being increased.
Unfortunately, the conventional hydraulic control circuit constructed as described above causes a problem of often failing to generate a sufficient pilot pressure when the first to third control valves V.sub.1 to V.sub.3 each are changed over. For example, when the second control valve V.sub.2 is changed over in order to forward tilt the tilt cylinder while the amount of fluid discharged from the pump P is kept reduced, the weight of a fork of a forklift truck causes a negative pressure to be generated in each of the neutral flow passage 131 and parallel feeder 135. This leads to insufficient generation of the pilot pressure, resulting in failing to ensure full stroke of the second control valve V.sub.2.
The conventional hydraulic control circuit has another disadvantage. More particularly, when the proportional solenoid is suddenly operated and then stopped, a back pressure is instantaneously generated by the back pressure valve 133 to cause the actuator to be actuated a little. In this instance, when the neutral ports 125, 127 and 129 each fail to be completely closed at the time of sudden operation of the proportional solenoid, most of pressure oil is caused to be returned through the neutral flow passage 131 to the tank T, so that actuation of the actuator is interrupted.